1. Field of the Invention
The present invention relates to a semiconductor optical modulator of Mach-Zehnder type or the like.
2. Description of the Related Art
A technology relating to a p-i-n type semiconductor optical modulator is disclosed in Naoto Yoshimoto, Yasuo Shibata, Satoshi Oku, Susumu Kondo, and Yoshio Noguchi, “Design and Demonstration of Polarization-Insensitive Mach-Zehnder Switch Using a Lattice-Matched InGaAlAs/InAlAs MQW and Deep-Etched High-Mesa Waveguide Structure”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, No. 9, pp. 1662, SEPTEMBER 1999. The p-i-n type semiconductor optical modulator disclosed in the above-described non patent literature has an optical waveguide in which a phase of light is modulated. The optical waveguide includes a p-type cladding layer, an i-type core layer, and an n-type cladding layer. The i-type core layer corresponds to a core of the optical waveguide and is disposed between the p-type cladding layer and the n-type cladding layer. The i-type core layer has an i-type multiple quantum well (MQW) structure.
Light propagating in the optical waveguide is absorbed due to free-carrier absorption when free carriers (electrons in the conduction band or holes in the valence band) are present in the optical waveguide. In addition, as the free carriers increase or decrease in the optical waveguide, the refractive index in the optical waveguide is also changes in accordance with the Kramers-Kronig relation. Such a phenomenon is known as a plasma effect with respect to the free carrier.